Organic light emitting diodes (OLEDs) are being increasingly widely used in general lighting, for example as large-area luminous surfaces (surface light source). A conventional OLED includes on a substrate an anode and a cathode with an organic functional layer system between the anode and cathode. The organic functional layer system may include one or a plurality of emitter layer(s) in which electromagnetic radiation is generated, one or a plurality of charge generating layer structure(s) each composed of two or more charge generating layers (CGL) for charge generation, and one or a plurality of electron blocking layers, also designated as hole transport layer(s) (HTL), and one or a plurality of hole blocking layers, also designated as electron transport layer(s) (ETL), in order to direct the current flow. The OLED emission can easily be varied via the operating current of the OLED. Adaptation to external and internal light conditions is possible as a result.
The external light conditions can change (short-term process) e.g. if the OLED in a room with a window is exposed to the diurnally variable sunlight. Furthermore, the emitted wavelength spectrum of the OLED is subjected to aging processes (long-term process), such that, depending on the OLED layer construction and processing, the luminance typically decreases with time. The decrease in the luminance is brought about e.g. by increased current densities or temperatures which occur during the operation of the OLED and can damage the organic system. In order to keep constant the luminance in the environment of the OLED surface light source with time, the luminance in the OLED-illuminated room can be kept constant by means of manual dimming or external switched-on sensors with electronic circuit.
In one conventional method, sensors are applied to the OLED and collect the light of the OLED. In that case, however, the sensors are conventionally positioned such that they can observe only partial regions of the OLED, for example the edge or the region directly around the sensor. If, in the case of an OLED on a glass substrate, for example, the sensor is applied at the edge in order to measure the light propagating in the substrate, the light which reaches the sensor comes only from a region having a maximum width of 10 mm at the edge of the OLED. The remaining light emitted by the OLED and guided in the glass substrate is absorbed on the way to the sensor in the organic functional layer structure.